This invention relates to a printing ink suitable for publication gravure printing having a substantial portion of organic solvent replaced with water, the physical form of the ink being an organic solvent and water emulsion.
Inks consist of a colorant and a liquid vehicle, in which the colorant is dispersed or dissolved. After the ink is applied, a binder present in the vehicle adheres the colorant to the substrate on which it is printed. The binder may be a drying oil which oxidizes and is converted from a liquid to a solid film, or a resin dissolved or dispersed in a solvent, which forms a solid film binding the colorant to a substrate after the solvent is evaporated. Combinations of drying oils, resins and organic solvents are also employed.
The solvents used in gravure printing are primarily aromatic, aliphatic and naphthenic hydrocarbons. Additional organic solvents useful in gravure printing include alcohols, ketones, esters, ethers and halogenated compounds. Important criteria in solvent selection are solvent strength relative to a desired resin, evaporation rate, viscosity, cost, odor and toxicity, and compatibility with the substrate to be printed upon. Occasionally, small amounts of water, less than 5 wt. %, usually about 2 wt. %, are added to gravure inks to overcome printing problems.
Since the solvent typically constitutes from 30 to 70 wt. % of a gravure ink composition, the emission of volatile organic compounds (VOC) represents a significant environmental concern. In order to comply with myriad federal, state and local environmental regulations, gravure printers have been required to install solvent recovery systems. The systems represent both a substantial capital outlay and operating expense, and can limit production rates. Losses of organic solvent to the atmosphere also represent raw material make up expense.
Water based inks have been developed to avoid the environmental impact and operating expenses associated with organic solvents and, in some cases, reduce raw material costs. Water dispersible or soluble binders, such as polyacrylic acid and polymethacrylic acid, may be employed, as well as water miscible co-solvents. In Peters et al., U.S. Pat. No. 4,104,219, a polyalkyl acrylate binder is dispersed in a homogenous, aqueous phase. Defago et al., U.S. Pat. No. 4,543,102, disclose an aqueous ink composition having up to 8% of a water immiscible organic solvent, which contains polyvinyl alcohol and an acrylic polymer or copolymer.
A method of preparing an aqueous ink for ink jet printing is described in Ober et al., U.S. Pat. No. 4,692,188, comprising the steps of:
1. dissolving a polymer, such as polystyrene or polyester, and an oil soluble dye in a water immiscible solvent, such as methylene chloride; PA1 2. adding an aqueous phase and a surfactant; PA1 3. forming an emulsion; and PA1 4. evaporating the solvent to form an ink having particles of polymer and dye suspended in the aqueous phase.
All of the water immiscible organic solvent is removed prior to application of the ink to a substrate.
There are a number of drawbacks associated with water-base inks. The press handling characteristics of water-base inks are uniquely different from organic solvent base materials. Once dry, aqueous inks may be reluctant to redissolve in water, depending upon the kind of resin system used in the ink, resulting in an increase in cylinder plugging and number of press wash ups. Conversely, the resin system may not be sufficiently water resistant upon drying, and the ink may be easily smeared in the presence of moisture.
The substrate being printed upon strongly affects the performance of water-based inks. In applications such as printing on low surface tension film and publication gravure employing coated paper, organic solvent systems give superior performance at this time. Even with uncoated paper, water may cause dimensional distortion and curling.
The resin system used in water-based inks can give rise to poor dot spread and increased skipped dots. These problems may be difficult to overcome given the limited availability of resins for use in water-based systems. Accordingly, the adoption of a water-based system may require changes in gravure cylinder engravings, press operating and housekeeping procedures, and at higher press speeds, dryer modifications. The aqueous phase of water-based inks contains many additives such as surfactants and alcohol to reduce surface tension and to stabilize pigment-resin dispersions. Ammonia or other volatile amines are often added to aid in dissolving the resin. The effect of the aforementioned additives must be evaluated and if necessary, compensated for in the printing process.
Printing pastes for screen printing on textiles have been formulated based on oil-in-water emulsions. Uhl et al., U.S. Pat. No. 3,825,431, disclosed defoamers which are particularly useful in such print paste emulsions. A printing ink emulsion for lithographic printing is disclosed by DeSanto, Jr. et al., U.S. Pat. No. 4,981,517. Instead of applying a separate aqueous and oil based ink coat to the printing plate, an emulsion is provided which is applied in a single-application-step process. The emulsion obviates the requirement of a water coat application to the non-image area of a printing plate to render that area ink repellent.
Other uses of emulsions in printing ink formulations are disclosed by Krishnan et al., U.S. Pat. No. 5,098,478 and Carlick et al., U.S. Pat. No. 5,158,606. The rub resistance of an ink may be improved by incorporating an emulsion polymer into the composition. In Krishnan et al., the ink is water-based, whereas Carlick et al. disclose an oil based composition having up to 15 wt. % of an emulsion polymer incorporated therein.
Despite the extensive work in development of water-based ink formulations, there remains a strong need in the gravure printing industry for an ink having reduced VOC, which retains the performance standards of organic solvent based inks.